The present invention relates generally to apparatus for producing dry-pressed moldings from a particulate or granular molding material, such as a ceramic molding composition.
In particular the invention relates to apparatus of the type comprising basically a first mold half, a second mold half and a vacuum shooting head. In such apparatus, with the second mold half removed, the first mold half and the vacuum shooting head can be brought together to define a loading cavity into which the molding composition can be drawn by applying a vacuum. The molding process employed with such apparatus comprises the steps of introducing the molding composition into the loading cavity where it is precompressed into a premolding, bringing the second mold half into alignment with the first mold half after the vacuum shooting head has been removed to leave the premolding behind in the first mold half, and pressing the premolding between the first and second mold halves to form the molding.
Apparatus of this general type used in such a process is known from German Offenlegungsschrift No. 3,101,236, which describes and illustrates apparatus in which the first mold half is designed for isostatic pressing. The essential point in isostatic pressing is that, after the first and the second mold halves have been brought up to each other, a fluid under pressure is introduced behind a press member belonging to one or other of the mold halves. The press member defines the shape of the molding surface of the associated mold half and presses the molding against the other mold half which, if desired, may also be equipped with an isostatic press membrane. As far as the present invention is concerned, however, the two mold halves may be equipped with rigid molding surfaces or with molding surfaces which are formed by elastomeric layers supported so as to be dimensionally rigid during pressing, in which case the surface of the vacuum shooting head which faces into the loading cavity, and the molding surface of the second mold half, are differently shaped in such a way that, despite a varying thickness of molding composition in the loading cavity, that is despite a varying wall thickness of the premolding, an approximately constant degree of compression is obtained in the finished molding as a result of the fact that the compression-molding process starts in the zones where the premolding is relatively thick before it starts in the zones where the premolding is relatively thin.
The procedure described in the above mentioned German Offenlegungsschrift No. 3,101,236 is particularly suitable for processing ceramic materials, such as porcelain, but is also suitable for processing other dry, free-flowing particulate materials, such as coal-containing and metallic molding compositions. In the field of ceramics the process is particularly suitable for producing moldings which are not rotationally symmetrical, that is moldings which cannot be produced on traditional potter's wheels and which therefore have hitherto traditionally been produced by pouring liquid slip into moisture-absorbent hollow plaster molds. The procedure described in the above-mentioned German Offenlegungsschrift No. 3,101,236 offers a significant increase in productivity over the traditional casting technique.